Applications of tracking an object, within a volume of interest, are known in the art. For example, tracking a helmet, worn by a pilot in a cockpit is used to determine the location and orientation that the pilot is looking at (i.e., by further determining the gaze direction of the eye of the pilot). Tracking a catheter, within a body of a patient, may be used to display a representation of the catheter on an image of the body (e.g., an X-Ray image, CT image, MRI image, PET image and the like). Tracking various body parts of a person may be used to emulate the movement of that person in a virtual reality environment.
Systems for tracking an object, within a volume of interest using magnetic fields, are known in the art. These systems are referred to herein as “magnetic tracking systems”. Magnetic tracking systems track the object by repeatedly determining the location and orientation of the object, in the volume of interest, relative to a coordinate system associated with the magnetic tracking system. The term “pose” refers hereinafter to either location, orientation or both. The term “location” relates to the coordinates of an object (i.e., according to a determined coordinate system such as X, Y, Z) and the term “orientation” relates to the direction of the object in the determined coordinate system (e.g., Eulers angles). The term “magnetic coordinate system” refers hereinafter to a coordinate system associated with the magnetic tracking system. In general, magnetic tracking systems employ a magnetic field transmitter and a magnetic field detector. The magnetic field transmitter may employ several magnetic field generators (e.g., coils with electric current flowing there through). The magnetic field detector may employ several magnetic field sensors (e.g., coils with electric current induced therein, hall-effect sensors). In general, for the purpose of magnetic tracking, the number of generators times the number of sensors should at least equal the number of required location and orientation parameters (e.g., the number of required location and orientation parameters may be six, three for location and three for orientation). According one configuration of magnetic tracking systems, the magnetic field transmitter is fixed at a known pose in the volume of interest and the magnetic field detector is mounted on the tracked object. According to another configuration of magnetic tracking systems, the magnetic field transmitter is mounted on the tracked object and the magnetic field detector is fixed at a known pose in the volume of interest.
To determine the position of the tracked object, within a volume of interest, using magnetic fields, the amplitude and direction of the magnetic field at each location in the volume of interest should be known (i.e., either measured or computed). The amplitude and direction of the magnetic field is referred to hereinafter as the “magnetic field vector”. The ensemble of magnetic field vectors at corresponding locations in the volume of interest is referred to hereinafter as the “magnetic field map”. The magnetic tracking system determines the pose of a tracked object by measuring the magnetic flux at that pose. The magnetic tracking system determines the magnetic field vector according to the measured magnetic flux, and determines the pose corresponding to that magnetic field vector according to the magnetic field map.
A magnetic field map may have one of several forms. Accordingly, the magnetic field map may have the form of a physical model relating each location in the volume of interest with an amplitude and direction of the magnetic field. The physical model includes physical parameters. For example, when the magnetic field is generated by a coil, the magnetic field model may be that of a dipole with physical parameters such as coil radius and the number of turns of the coil. Alternatively, the magnetic field map may have the form of a mathematical model, without any knowledge of the physical parameters of the magnetic field (e.g., polynomial, spline). According to yet another alternative, the magnetic field map may be in the form of a Look Up Table (LUT) associating between a selected number of known locations in the volume of interest with corresponding values of the magnetic field vectors at these selected locations. The value of the magnetic field vectors, between entries in the LUT, is determined according to an interpolation scheme (e.g., an interpolation function such as a straight line, a sinc function etc).
The magnetic field map may be determined at the manufacturing stage of the magnetic field transmitter. However, such a map does not allow for all the interferences introduced to the magnetic field in a specific volume of interest (e.g., interferences caused by ferromagnetic objects or other electromagnetic transmitters within the volume of interest). The magnetic field map may be determined, prior to tracking, individually for each volume of interest. This map includes the interferences (i.e., when those exist) introduced to the magnetic field in the volume of interest (e.g., due to metallic objects present in the volume of interest). Accordingly, the magnetic field transmitter is activated and the magnetic field detector is moved through a plurality of known poses in the volume of interest. The magnetic field detector measures the magnetic field vector at each known location. A processor processes these measurements and produces the magnetic field map. When the magnetic field map is a physical model or a mathematical model, the processor estimates the parameters (i.e., the physical parameters or the mathematical parameters) to determined the model that best fits the measurements. When the magnetic field map is a LUT, the processor constructs the LUT according to the measurements and the known locations. It is noted that the term “mapping” refers to herein after to determining the magnetic field map. The terms “magnetic field model” and “model” will be used herein interchangeably.
Additionally, when the magnetic tracking system is required to determine the pose of the tracked object in a coordinates system associated with the volume of interest, the magnetic tracking system registers the magnetic field map with the coordinate system associated with the volume of interest. The coordinate system associated with the volume of interest is referred to herein as the “reference coordinate system”. The term “registering” refers to determining a correspondence between the poses relative to the magnetic coordinate system and the poses relative to the reference coordinate system. This reference coordinate system is, for example, the coordinate system of the airplane, the coordinate system of a virtual environment or the coordinate system of a medical image. Thus, the location and orientation of the tracked object is known relative to the reference coordinate system. The magnetic tracking system registers the magnetic field map with a reference coordinate system for example, by placing the magnetic field detector at a known pose relative to the reference coordinate system and determines the pose of the magnetic field detector relative to the magnetic coordinate system. Alternatively, when the pose of the magnetic field transmitter, relative to the reference coordinate system, is known, each pose relative to the magnetic coordinate system is associated with a respective pose relative to the reference coordinate system.
The publication to Livingston et al., entitled “Magnetic Tracker Calibration for Improved Augmented Reality Registration”, directs to a system and a method for mapping a magnetic field using LUT and for registering the magnetic field map with a reference coordinate system. According to Livingston et al., a magnetic tracking system tracks the pose of a receiver attached to the object being tracked. However, metal and electromagnetic devices (e.g., computers, Cathode Ray Tubes, metal objects and electrical wirings) distort the field created by the transmitter. Therefore, the magnetic field model, used by the magnetic tracking system, may be inaccurate. Thus, the system to Livingston et al maps the magnetic field and determines correction factors for each location in the volume of interest. Accordingly, the receiver is attached to six degrees of freedom mechanical arm tracker, which determines a vector of locations and orientations of the tip of the arm relative to the base of the arm. The coordinates system associated with the mechanical tracking systems serves as the reference coordinate system. Thus, each pose determined by the magnetic tracking system, has a pose determined by the mechanical tracking system associated therewith. The differences between these associated poses are used to determine the corrections needed for the poses determined by the magnetic tracking system.
U.S. Pat. No. 5,847,976 to Lescourret, entitled “Method to Determine the Position and Orientation of a Mobile System, Especially the Line Of Sight in a Helmet Visor”, directs to analytic modeling of electromagnetic fields. These fields include a first electromagnetic field created by a transmitter, a second field created by eddy currents induced in metal object within the volume of interest by a first field and a third field created by currents induced in the tracked object (e.g., a helmet of a pilot) by the first and second fields. Each one of the three fields is characterized independently of the other fields by the coefficients of a model associated with each field.
The first field is determined by measuring the field created by the transmitter in free space. The field is measured at points of measurements by translating a mechanical system bearing the sensor through these points. The parameters of a model of this field are estimated.
The second field is determined by measuring the field within the volume of interest including the metal objects. The field is measured at points of measurements by translating a mechanical system bearing the sensor through these points. The parameters of a combined model including both the first and the second field are estimated. The model of the first field is subtracted from this combined model.
The third field is determined by first plunging disturbance sources into the magnetic field produced by the transmitter. The model of the disturbance due to each disturbance source, at the sensor, is modeled as an explicit function of the existing mean field at the point of origin of a coordinate system defining this source. Thus, the model of each source depends explicitly on the field into which each source is plunged into. In a second stage, the sensor is plunged into the magnetic field and the disturbance caused by each source is determined by its model and of the mean magnetic field. In a third stage, disturbances due to the sources are summed. Finally, in a fourth stage, this sum is deducted from the measurement made by the sensor. In this way, all the parameters of the source model representing the phenomenon of disturbance produced by this source are independent of the field into which the sensor and all the sources are plunged.
The publication entitled “A Framework for Calibration of Electromagnetic Surgical Navigation Systems” to Wu et al directs to employing a 3D optical navigation system to calibrate the measurement distortion of a magnetic tracking system. To that end the publication to Wu et al directs to employing a Lego robot which moves semi-statically within the desired calibration space with infrared tracking markers and the magnetic tracking sensors attached thereto. The calibration process includes three steps, registration between the magnetic tracking coordinate system and the optical tracking coordinate system and constructing an error field of the magnetic tracking system and error correction and validation. The coordinate system of the optical tracking system is employed as the “ground truth”. The error in Position error and orientation error can be expressed as 3D vectors. The publication to Wu et directs to two method to express the 3D error vectors in space. One is the KD trees and the other is fitting Bernstein polynomials. According to the publication to Wu et al, the magnetic field distortion can be characterized by a Bernstein polynomial of the fourth order.
European patent 2 535 021 to Ophir et al directs to a system for mapping a magnetic field in a volume of interest which includes a magnetic field transmitter, a freestanding magnetic field detector and a processor. The processor is coupled with the magnetic field detector. The magnetic field transmitter generates a magnetic field in the volume of interest. The freestanding magnetic field detector is operative to freely move within the volume of interest. The freestanding magnetic field detector acquires measurements of the flux of the magnetic field at a plurality of poses. The processor estimates theses poses employing a stored generic magnetic field model. The processor re-estimates parameters characterizing the magnetic field model according to deviations between the measurements of the magnetic field and according to predictions of the flux. The predictions are determined according to a stored generic magnetic field model related to the environment. Thereby the processor estimates a new magnetic field model. In the system directed to by Ophir et al, the number of the generators times the number of the sensors is larger than the number of degrees of freedom required to track an object in the volume of interest. This provides additional information relating to the deviations between the magnetic field predicted by the stored magnetic field model and the measurements of the flux of the magnetic field. The processor employs the additional information when re-estimating the parameters characterizing the magnetic field model.